In general, in the case of a boiling-water reactor, for a neutron measurement apparatus that measures neutrons inside the reactor during startup or shutdown, pulse counting method and Campbell method are used in combination.
As such a neutron measurement apparatus, there has been a start-up range monitor (also referred to as a wide range monitor). Refer to U.S. Pat. No. 4,493,811, for example, the entire content of which is incorporated herein by reference.
In the above-described start-up range monitor, two different measurement methods, or the pulse counting method and Campbell method, are used in combination. Therefore, it is required to smoothly connect measurement results in such a way as to avoid a shift in the linearity of the input and output at a point where the two measurement methods are switched.
FIG. 15 is a schematic graph that is obtained by using the pulse counting method and Campbell method to measure an output signal of a neutron detector, showing relation of measured values with respect to neutron intensity when there is a Transition region. In FIG. 15, a range where a portion of curve A, which represents the results of measurement by pulse counting method, that keeps the input-and-output linearity with respect to neutron intensity overlaps with a portion of curve B, which represents the results of measurement by Campbell method, that keeps the input-and-output linearity with respect to neutron intensity is referred to as a Transition region of the pulse counting method and Campbell measurement method.
The neutron detector generates one pulse-like signal (pulse signal) for one neutron entering the detector. However, if a larger number of neutrons start entering the neutron detector as a result of an increase in the neutron intensity, the neutron detector would face difficulty counting each of individual pulse signals. As a result, a so-called pulse-counting saturated state would emerge. In this manner, the curve A loses linearity when the neutron intensity is increased.
Meanwhile, according to the measurement by Campbell method, the measurement is carried out by calculating the mean square of AC signal component of a neutron detector output signal that is made up of overlapping pulse signals. However, in a range where the neutron intensity is low, an AC component of background is stronger than the AC signal component of the neutron detector signal. Accordingly, the curve B loses linearity in the range where the neutron intensity is low.
Upper-limit neutron intensity Φ1, to which from lower region the linearity is maintained in the case of the pulse counting method, needs to be sufficiently higher than lower-limit neutron intensity Φ2, to which from upper region the linearity is maintained in the case of the Campbell method. If not, a sufficiently-wide Transition region cannot be formed. As a result, when the measurement method is switched from the pulse counting method to the Campbell method, it is difficult to smoothly connect the measured values.
FIG. 16 is a schematic graph that is obtained by using the pulse counting method and Campbell method to measure an output signal of a neutron detector, showing relation of measured values with respect to neutron intensity when there is a discontinuous region. As shown in FIG. 16, if upper-limit neutron intensity Φ1, to which from lower region the linearity is maintained in the case of the pulse counting method, is lower than lower-limit neutron intensity Φ2, to which from upper region the linearity is maintained in the case of the Campbell method, there will be a discontinuous region between the Pulse and Campbell regions where the linearity can be achieved by neither the pulse counting method nor Campbell method. As a result, a difference in level emerges when the measurement results of the two are connected. In this manner, it is difficult to smoothly connect the curves A and B.
The object of embodiments of the present invention is to enable the measurement by Campbell method of relatively low-intensity neutrons, which have been measured by pulse counting method in the conventional case.
According to an aspect of the embodiment, there is provided a neutron measurement apparatus to measure neutron intensity in a region where a nuclear reactor power is lower than the Power Range which is close to a rated reactor power, the apparatus comprising: a neutron detector to generate an output signal corresponding to an incoming neutron; a preamplifier to amplify an output signal of the neutron detector to output a preamplifier to amplify an output signal of the neutron detector to output the amplified output signal of the neutron detector as a neutron detection signal; a pulse counting unit to measure the neutron intensity by using pulse counting method by which the neutron detection signal is used to count the number of individual pulses; a Campbell measurement unit to measure the neutron intensity by using Campbell method by which a time average of squares of AC component of the neutron detection signals is calculated; a correction constant calculation unit to calculate a correction constant for correcting an output of the Campbell measurement unit by using an output of the pulse counting unit and an output of the Campbell measurement unit; and a correction value calculation unit that outputs, based on the output of the Campbell measurement unit, a corrected value by using the correction constant.
According to another aspect of the embodiment, there is provided a neutron measurement for measuring neutron intensity in a region where a nuclear reactor power is lower than the Power Range which is close to a rated reactor power, the method comprising: a pulse counting measurement step of measuring the neutron intensity by using pulse counting method; a correction constant calculation step of calculating a correction constant based on relation between an output of a pulse counting unit and an output of a Campbell measurement unit, in a region where linearity of input and output is maintained in the pulse counting method; and a correction value calculation step by a correction value calculation unit of receiving, as an input, an output value of the Campbell measurement unit after the correction constant calculation step, and calculating a corrected output value of the Campbell measurement unit by using a correction formula that uses the correction constant.